Variable volume test chamber

ABSTRACT

A testing apparatus which has a test chamber located within a chamber frame. The apparatus includes a computer controlled ergonomic system which can vary the temperature of the test chamber. Within the test chamber is an adjustable ceiling that can be moved to vary the volume of the chamber. Extending around the outer edge of the ceiling is an inflatable seal which seals the chamber when the ceiling is moved to a new position. The ceiling and walls have a plurality of adjustable vents that direct fluid flow into the chamber. The doors of the apparatus are each constructed from an inner wall that is coupled to an outer wall by a plurality of floating joints. The floating joints allow the inner wall to contract or expand relative to the outer wall without creating a corresponding stress on the outer wall or the joints. To prevent an operator from prematurely opening a door after temperature cycling, the door locks are computer controlled so that the doors cannot be unlocked until the test chamber reaches an operator safe condition. The computer also prevents the ceiling from being adjusted until the front doors are opened and the table is pulled out of the chamber. One door also has an override button that allows the operator to open the door from the inside of the test chamber. Additionally, the testing apparatus includes a vibration table which can be pulled to a position outside of the test chamber. The table has a brake and shock absorbers that prevent an undesirable shock to the test parts when moving the table into and out of the test chamber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a test chamber that can perform thermaland vibrational test on an object located within the chamber.

2. Description of Related Art

It is often desirable to test a component or assembly under simulatedenvironmental conditions. For example, prototype electronic assembliesare exposed to various temperature, vibration and shock loads todetermine whether the assembly can operate within its intendedenvironment. Environmental testing is typically performed within a testchamber that is coupled to an ergonomic system which can vary thetemperature and humidity within the chamber. The size of the test partcan vary considerably, ranging from a large aircraft component to asmall semiconductive device. The test chamber must be large enough toaccommodate all types of test parts.

Using a large test chamber creates a relatively significant mass of airwithin the chamber. When temperature cycling a test part, the large airmass and inner chamber surfaces create a significant thermal inertiawhich must be overcome by the ergonomic system, resulting in additionalwork for the system and limiting the response time of the chamber. Sometest chambers incorporate partition walls that reduce the volume of thetest chamber. Assembling and disassembling the partition walls requiresvaluable testing time. It therefore would be desirable to provide anautomated variable volume test chamber.

Thermal cycling a test part within a test chamber may result in atemperature differential between the test chamber and the room of morethan 100° C. A large temperature differential may cause the inner wallsof the test chamber to expand or contract an amount different than theouter walls of the test chamber. In particular, if the inner chamber isat a sub-zero temperature the inner wall of the door will contract morethan the outer wall of the door. It has been found that extreme thermalcontractions/expansions of the inner wall will warp the door and breakthe seal between the chamber and the door. A thermal cycle of the doormay also cause the door fasteners to fail. It would be desirable to havea test chamber door that will not warp or become damaged when subjectedto thermal cycling.

After a part is tested, it is removed from the test chamber. It has beenfound that the operator may become injured or the test part may becomedamaged, if the test chamber is opened before the test is complete. Forexample, the operator may become burned if he enters the test chamberwhile at an elevated temperature. Likewise, opening the test chamber ata subzero temperature may result in condensation forming on, or withinthe test part and damaging the same. It would be desirable to provide atest chamber unit which does not allow the access door to be openedbefore the chamber reaches an operator "safe" condition. Additionally,it would also be desirable to allow an operator to open the access doorfrom within the test chamber in the event the operator becomes locked inthe chamber.

To apply vibration and shock loading, the test parts are typicallymounted to a vibration table located within the test chamber. Because ofthe limited space of a test chamber, it can be difficult to mount thetest part onto the table within the chamber. Therefore it would bedesirable to provide a vibration table that can be pulled to a positionoutside of the test chamber.

SUMMARY OF THE INVENTION

The present invention is a testing apparatus which has a test chamberlocated within a chamber frame. The apparatus includes a computercontrolled ergonomic system which can vary the temperature of the testchamber. Within the test chamber is an adjustable ceiling that can bemoved to vary the volume of the chamber. Extending around the outer edgeof the ceiling is an inflatable seal which seals against the chamberwhen the ceiling is located at one of its positions and the doors areclosed. The ceiling and walls have a plurality of adjustable vents thatdirect conditioned air into the chamber. The doors of the apparatus areeach constructed from an inner wall that is coupled to an outer wall bya plurality of floating joints. The floating joints allow the inner wallto contract or expand relative to the outer wall without creating acorresponding stress on the outer wall or the joints. To prevent anoperator from prematurely opening a door after temperature cycling, thedoor locks are computer controlled so that the doors cannot be unlockeduntil the test chamber reaches an operator "safe" condition. Thecomputer also prevents the ceiling position from being adjusted untilthe front doors are opened and the table is pulled out of the chamber.One front door also has an override button that allows the operator toopen the door from the inside of the test chamber. Additionally, thetesting apparatus includes a vibration table which can be pulled to aposition outside of the test chamber. The table has a brake and shockabsorbers that prevent an undesirable shock to the test parts whenmoving the table into and out of the test chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the present invention will become morereadily apparent to those ordinarily skilled in the art after reviewingthe following detailed description and accompanying drawings, wherein:

FIG. 1 is a perspective view of a testing apparatus of the presentinvention;

FIG. 1a is a rear perspective view of the test apparatus;

FIG. 2 is a cross-sectional view of the test chamber showing anadjustable ceiling;

FIG. 2a is a cross-sectional view of the ceiling seal;

FIG. 3 is an enlarged view of a jack screw assembly;

FIG. 4 is a top sectional view of a test chamber ceiling;

FIG. 4a is a top sectional view of the adjustable ceiling;

FIG. 5 is a side view of an inner wall of the test chamber;

FIG. 6 is a front view of a vent;

FIG. 7 is a cross-sectional view of the vent of FIG. 6;

FIG. 7a is an enlarged view of a vent vane end tab attachment;

FIG. 8 is a rear view of an inner wall of the door;

FIG. 9 is a rear view of an outer wall of the door;

FIG. 10 is an enlarged view of a rivet joint;

FIG. 11 is a cross-sectional view of a table;

FIG. 12 is an end view of the table;

FIG. 13 is a view similar to FIG. 11 showing the table pulled out of atest chamber;

FIG. 14 is a side view of the table with a handle in a verticalposition;

FIG. 15 is a side view of a brake assembly of the table;

FIG. 16 is an enlarged end view of the brake assembly;

FIG. 17 is a view similar to FIG. 14 showing the handle rotated;

FIG. 18 is a side view of an automatic door locking assembly;

FIG. 19 is a flowchart showing a routine for performing a test andopening the doors of the test apparatus;

FIG. 20 is a side view of an emergency door button that can be operatedfrom the inside of the test chamber.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings more particularly by reference numbers, FIGS.1 and 1a show a testing apparatus 10 of the present invention. Thetesting apparatus 10 includes a test housing 12 and an ergonomic systemconsole 14. The housing 12 has a pair of front doors 16 and a rear door17 which provide access to a test chamber. The ergonomic system 14contains a computer and other components required to perform variousenvironmental tests within the test chamber of the housing 12. Forexample, the ergonomic system 14 may contain heating and coolingcomponents that raise and lower the temperature within the test chamber.The system 14 may also have equipment to increase and decrease thehumidity of the test chamber.

As shown in FIG. 2 the housing 12 has an adjustable ceiling 18 locatedwithin the test chamber 20. The ceiling 18 can move relative to thefloor 22 and walls 24 of the housing to vary the volume of the testchamber 20. The floor 22 may be a vibration table. The apparatus iscapable of simultaneously vibrating a test part and varying thetemperature within the chamber. The computer typically controls themovement of the ceiling 18. In the preferred embodiment, the ceiling 18will not move until the front doors are opened and the vibration tableis pulled out of the chamber. The doors and table may have switches thatprovide the computer with feedback signals when the front doors are openand the table is pulled out of the test chamber.

As shown in FIG. 2a, extending around the outer edge of the ceiling 18is an inflatable seal 26. The seal 26 is typically constructed fromsilicone and coupled to a source of pressurized air. The seal 26 isdeflated before the ceiling 18 is moved and inflated when the ceiling 18reaches a new position and the doors 16-17 are closed. The housing 12may have a valve (not shown) that automatically controls the flow of airinto and out of the seal 26. The doors 16-17 may also have inflatableseals (not shown) that expand when the doors 16-17 are closed anddeflate before the doors 16-17 are opened.

As shown in FIG. 3, the ceiling 18 is coupled to a number of jack screws28 that are mounted to flanges 30. The ceiling 18 has internal threadsthat cooperate with the jack screws 28 to move the ceiling when thescrews 28 are rotated. As shown in FIG. 4, the jack screws 28 arerotated by a reversible electric motor 32. The motor 32 is coupled tothe screws 28 by gears 34 and 36 and chains 40 and 42. Each jack screw28 may have a pair of outer flexible boots 44 that prevent contaminantsfrom affecting the operation of the screws 28. Each jack screw 28 mayalso have a bearing 46 that couples the end of the screw 28 to theflange 30.

As shown in FIG. 5, the housing 12 may have a sensor assembly 47 thatcontains a plurality of position sensors 48-50 that provide feedbacksignals when the ceiling is adjacent to one of the corner seals 51-53shown in FIG. 2. Referring to FIGS. 4 and 5, the sensor assembly 47 iscoupled to one of the jack screws 28 by gears 36 and 41, and chain 43.The feedback signal is typically provided to the computer of theergonomic system 14 which also controls the operation of the electricmotor 32 and the air valve of the seal 26. The console 14 may allow theoperator to remotely move the ceiling to one of a plurality of locationswithin the test chamber. By way of example, the third sensor 50 maycorrespond to a small chamber, the second sensor 49 may correspond to anintermediate chamber and the first sensor 48 may correspond to theceiling position of a large chamber.

If the operator selects the intermediate chamber, the computer willdetermine the present position of the ceiling based on the feedbacksignals of the sensors 48-50. The seal 26 is deflated and the electricmotor 32 is energized to rotate the jack screws 28 and move the ceiling18. The screw 28 rotation continues until the ceiling reaches the secondsensor 49. When the ceiling is in position, the second sensor 49provides a feedback signal to the computer which de-energizes theelectric motor 32 and inflates the seal 26 when the doors 16-17 areclosed.

As shown in FIG. 4a, the ceiling 18 typically has 9 ducts 54. Six of theducts 54 typically provide conditioned air, while the remaining ducts 54provide a return path from the test chamber. Referring to FIG. 2, theducts 54 are coupled to the ergonomic system by bellows 56. The bellows56 provide flexible ducts for the ceiling 18. As shown in FIG. 5, thewalls have a plurality of vents 58. The vents 58 are located within thewalls 24 so that fluid flow can occur into the test chamber when theceiling is in the low, intermediate and high positions. The vents 58 arealso arranged so that a partition wall (not shown) can be attached tothe ceiling 18 to further reduce the volume of the test chamber 20.

FIGS. 6, 7 and 7a show a preferred embodiment of the vents 58. The vents58 each have a throttle plate 60 coupled to a housing 61 by a set screw62. Both the throttle plate 60 and the main housing 61 have slots 63that regulate air flow between an ergonomic duct (not shown) and thetest chamber. The throttle plate 60 can be rotated by a tool 64 that isinserted into an allen cup in the set screw 62. Rotating the throttleplate 60 varies the alignment of the slots 63 between the housing 61 andthe plate 60, and the flow into the chamber. The plate 60 may beseparated from the housing 61 by a Teflon washer 65 and clamped togetherby a nut 66 and a wavy spring 67.

Each vent 58 may also have a vane plate 68 that is coupled to the setscrew 62 by nut 69 and spring 70. Coupled to the vane plate 68 are aplurality of vanes 71. As shown in FIG. 7a, the vane plate 68 istypically constructed from two separate pieces that overlap to createopenings 72 which receive outer tabs 73 of the vanes 71. The vane tabs73 each have a roll pin 74 that presses the vane 71 into a pair ofwashers 75 and a wavy spring 76. The washer/spring assembly presses intoa bushing 77 that extends into the opening 72. The wavy spring 76provides a force that maintains the position of the vane 71. Each vanecan be individually rotated to provide directional air flow control intothe test chamber. Additionally, the vane plate 68 can be rotatedrelative to the housing 61 to change the direction of fluid flow. Asshown in FIGS. 8-10 the access doors 16 each have an outer wall 82 andan inner wall 80. The walls are joined together by a plurality offlexible joints 84 located about the door. The flexible joints 84 eachcontain a rivet 86 that is attached to the inner wall 80 and an innerfastener member 88. The rivet 86 extends through a clearance hole 90 ofan outer wall flange 92. The flange 92 is separated from the inner wall80 and the fastener member 88 by a pair of washer members 94. The rivet86 squeezes the outer wall flange 92 between the inner wall 80 and thefastener member 88, thereby attaching the outer wall 82 to the innerwall 80.

A change in temperature within the test chamber will cause the innerwall 80 to either expand or contract. The clearance hole 90 of the outerwall flange 92 allows the inner wall 80 to move relative to the outerwall 82. The relative wall movement eliminates warpage of the inner wall80, the outer wall 82 and stress on the rivets 86.

As shown in FIGS. 11-13, the testing apparatus includes a vibrationtable 100. The vibration table 100 is mounted to a table supportassembly 101. The test parts are typically mounted to the vibrationtable 100. To make the vibration table 100 more accessible to theoperator, the support assembly 101 is constructed to be pulled out ofthe test chamber. The support assembly 101 has a plurality of rollers102 that slide within two pairs of telescopic channels 104 located onslide supports 106 that are part of a table base 107 mounted to thechamber frame. The slides have a stop that prevents the table from beingdetached from the housing.

The support assembly 101 also has a retractable wheel assembly 108. Thewheel assembly 108 includes a follower 110 that is pivotally connectedto the assembly 101. The follower 110 has a roller 112 which moves alonga track 114. The follower 110 is also connected to two wheels 116 bylinkage bar 118. The wheels 116 are pivotally connected to the supportassembly 101. The track 114 has a step 120 that moves the roller 112 inan upward direction when the table 100 and support assembly 101 arepulled out of the test chamber. The upward movement of the slide 112induces a clockwise rotation of the follower 110 so that the linkage arm118 pulls the wheels 116 down. Pushing the table 100 and supportassembly 101 back into the test chamber generates a counterclockwiserotation of the follower 110 and a corresponding upward movement of thewheels 116.

As shown in FIGS. 14 and 15, the support assembly 101 is coupled to achain belt 122 that is coupled to a pair of sprockets 124 by anattachment link 126. The sprockets 124 are mounted to support brackets127 that are attached to the chamber frame. Movement of the table 100and assembly 101 rotates the chain 122 about the sprockets 124. As shownin FIG. 16, one of the sprockets 124 is coupled to a brake 128. Thebrake 128 has a spring biased electronic clutch (not shown) thatnormally holds the sprocket 124 and prevents the table 100 from movingrelative to the brackets 127. Referring to FIG. 14, the table has aswitch 130 that is connected to the electronic clutch of the brake 128.The switch 130 is activated by movement of a handle 132 that is used tomove the table 100 and the assembly 101. When the handle 132 is in avertical position, the switch 130 is open and the brake 128 preventsmovement of the table 100 and assembly 101.

As shown in FIG. 17, when the handle 132 is rotated, the switch 130 isclosed and the brake 128 is disengaged. Disengaging the brake 128 allowsthe sprockets 124 to freely rotate and the table 100 to move relative tothe base 107. The switch 130 and brake 128 provide a safety feature forthe table 100, wherein the table 100 will not move unless the operatoris holding a rotated handle 132. For example, if the operator is pushingthe table 100 into the test chamber and inadvertently releases thehandle 132, without the brake 28, the table would roll into the chamberand bump into the stops to produce an undesirable shock load on the testparts. When the push handle 132 of the present invention is released,the brake 128 will smoothly decelerate the table 100 and thereby preventpossible damage to the test parts and possibly the operator.

As shown in FIG. 15, the chain 122 has a pair of stops 34 and 136 thatengage corresponding shock absorbers 138 and 140 located adjacent to thesprockets 124. When the table 100 is pulled out of the test chamber, thestop 134 engages shock absorber 140 which reduces the velocity of thetable 100. Likewise when the table 100 is pushed back into the testchamber the stop 136 engages shock absorber 138 to slow down the table100.

As shown in FIG. 18, the testing apparatus may include an automatic doorlocking assembly 150. Each of the access doors 16-17 typically contain alocking assembly 150.

The locking assembly 150 has a pair of locking pins 152 located at thetop and bottom portions of the door. The locking pins 152 are coupled tocorresponding slots (not shown) in the test chamber frame to lock thedoors. The pins 152 are guided by guide bushings 154. The guide bushings154 are typically constructed from a low friction material such asacetal resin. The locking pins 152 are pivotally connected to a pair offirst linkage arms 156. The first linkage arms 156 are coupled to a pairof second linkage arms 158 by threaded joints 160. The length of theoverall assembly 150 can be adjusted by rotating the threaded joints160. The second linkage arms 158 are connected to a rotating plate 164that is coupled to the output shaft 166 of an actuator 168. The actuator168 is controlled by a control valve (not shown) that is connected tothe computer of the ergonomic system 14.

In the preferred embodiment, the actuator 168 is a spring return two-waypneumatic cylinder that can move the locking pins 152 between the lockedand unlocked positions. The control valve is typically a spring returnsolenoid valve that controls the flow of air into the actuator. When thesolenoid is energized by the computer, the valve is opened and theactuator 168 pulls the locking pins 152 into the unlocked position.De-energizing the solenoid reverses the flow of air to the actuator 168and forces the output shaft 166 and locking pins 152 to the lockedposition. The internal actuator spring will maintain the output shaft166 and the locking pins 152 in the locked position if the air pressureto the actuator is lost.

Referring to FIGS. 1 and 1a, an access door 16 and the housing 12 mayhave a pair of lights 170 and 172 and a single button 174. One of thelights 170 illuminates a red color which provides an indication that thechamber cannot be entered. The other light 172 illuminates a green colorwhich provides an indication that the chamber can be entered. Depressinga button 174 will unlock the doors when the chamber is safe to enter.When a button 174 is depressed, the computer is programmed to stop thetest being performed within the chamber and note where the test wasterminated. The ergonomic system will change the temperature of theinner chamber to an operator "safe" condition. Additionally, whenperforming thermal testing the chamber is typically filled with nitrogengas. The computer is programmed to purge the nitrogen and fill the testchamber with ambient air before the access doors can be opened. Afterpurging the test chamber with air, the computer is programmed to deflatethe seals and unlock the doors. The test chamber may containmicroswitches that detect when the doors are opened and provide afeedback signal to the computer to indicate the same.

FIG. 19 shows a flowchart of a routine typically performed by thecomputer and the test apparatus when performing a test. The routinedepicted can be performed by either hardware or software within thecomputer. The access doors are closed by the operator and the red lightis illuminated in processing block 200. The microswitches are sensed todetermine whether the doors are closed in decision blocks 201-203. Ifthe front doors 16 are open, the computer determines whether the table100 has been pulled out and the ceiling button (not shown) of theconsole has been depressed in decision blocks 204 and 205. If both ofthese conditions have been met, the computer moves the ceiling inprocessing block 206. If the doors are closed, the computer locks thedoors and inflates the seals in processing blocks 207 and 208,respectively.

The computer senses whether a button 174 has been pushed by the operatorin decision block 210. If the button has not been depressed, the chambersensors and the computer programmed algorithms determine, whether aprofile is running, the chamber is at an operator "safe" temperature andwhether an unsafe level of nitrogen is in the chamber, in decisionblocks 212, 214 and 216, respectively. If the results of the decisionblock are negative, the computer illuminates the green light in block218. The green light provides an indication to the operator thatalthough the doors are closed, the test chamber is safe to enter. If thetest chamber is not operator "safe", the computer turns on the red lightin block 220.

If a button 174 is depressed, the computer programmed algorithmsdetermine if a profile is running in decision block 222. A running testis stopped and the profile is saved in the memory of the computer inblock 224. The computer will be programmed to determine if the chamberis within predetermined temperature limits in decision block 226. If thetemperature of the test chamber is outside acceptable limits, theergonomic system will adjust the temperature in processing blocks 228.The computer will also control the level of nitrogen in the test chamberby determining whether the chamber contains too much nitrogen indecision block 230, and purging the chamber with air in block 232.

Once the test chamber is set to safe conditions, the green light isilluminated, the seals are deflated and the doors are unlocked inprocessing block 234, 236 and 238, respectively. The doors 16-17 arethen opened by the operator, an event that is sensed by the computer indecision blocks 240-242.

As shown in FIG. 20, the test apparatus may have an override button 250that allows an operator to exit the test chamber from inside thechamber. The button 250 is coupled to a plunger 252 which can switch theactuator 168 so that the output shaft 166 and locking pins 152 move intothe unlocked position. The button 250 is coupled to the plunger 252 bycam 254 which is pivotally attached to the door 16 In the event the testchamber has no compressed air or electricity, the button 250 may also becoupled to a manual override assembly that unlocks the door. Theoverride assembly includes the cam 254 that is pivotally attached to thedoor 16 and the button 250. Fastened to the cam 254 is a chain 256 thatis coupled to the rotating plate 164 of the lock assembly. Pushing thebutton 250 further will rotate the cam 254, pull the chain 256 and movethe locking pins 152 into the unlocked position. The override assemblymay have a spring 258 that returns the cam 254 when the button 250 isreleased. The automatic lock and override system increase the safety ofthe test apparatus for the operator.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art.

What is claimed is:
 1. A test chamber, comprising:a housing that has afloor and four walls which define an inner chamber, said inner chamberhaving a volume; a movable ceiling located within said inner chamber; amovement mechanism that moves said ceiling relative to said floor andvaries the volume of said inner chamber; and, a plurality of vents and aplurality of bellows that couple said vents to a source of air.
 2. Thetest chamber as recited in claim 1, further comprising an inflatableseal that seals said ceiling to said test frame.
 3. The test chamber asrecited in claim 1, wherein said movement mechanism includes a pair ofjack screws coupled to said ceiling and a motor to turn said jack screwsand move said ceiling.
 4. The test chamber as recited in claim 1,wherein said vents direct air into said inner chamber in at least twodifferent directions.
 5. The test chamber as recited in claim 1, furthercomprising a plurality of feedback devices that provide feedback signalswhich correspond to a position of said ceiling within said innerchamber.
 6. The test chamber as recited in claim 1, further comprising atable located within said inner chamber, said table being coupled to atrack so that said table can be pulled out of said inner chamber andfurther having a retractable wheel that supports said table.
 7. The testchamber as recited in claim 6, wherein said table has a brake thatreduces a velocity of said table as said table is pulled out of saidinner chamber.
 8. The test chamber as recited in claim 6, wherein saidtable has shock absorbers that reduce a velocity of said table when saidtable reaches a retracted position and an extended position.
 9. The testchamber as recited in claim 1, wherein said housing includes a door thatprovides access to said inner chamber, said door has an inner wall andan outer wall that are coupled together by a fastener.
 10. The testchamber as recited in claim 1, wherein said housing includes a door thatprovides access to said inner chamber, said door having a lock that canbe operated from within said inner chamber.
 11. The test chamber asrecited in claim 1, wherein said lock is coupled to a computer thatmaintains said lock in a closed position until said inner chamber is ina safe condition.
 12. A test chamber, comprising:a housing which has aplurality of walls that define an inner chamber and a door that providesaccess to said inner chamber, said inner chamber having an inner volumewith an inner chamber temperature; a movable ceiling within said innerchamber; a movement mechanism that moves said ceiling relative to saidfloor and varies the volume of said inner chamber; a vibration tablelocated within said inner chamber an ergonomic system that varies thetemperature of said inner chamber from an ambient temperature; and, acomputer that controls said ergonomic system.
 13. The test chamber asrecited in claim 12, wherein said ceiling has an inflatable seal. 14.The test chamber as recited in claim 13, wherein said movement mechanismincludes a pair of jack screws coupled to said ceiling and a motor toturn said jack screws and move said ceiling.
 15. The test chamber asrecited in claim 14, wherein said ceiling has a plurality of vents and aplurality of bellows that couple said vents to a source of air.
 16. Thetest chamber as recited in claim 15, wherein said vents direct air intosaid inner chamber in at least two different directions.
 17. The testchamber as recited in claim 16, further comprising a plurality offeedback devices that provide feedback signals which correspond to aposition of said ceiling within said inner chamber to said computer. 18.The test chamber as recited in claim 16, further comprising aretractable wheel that supports said table when said table is pulled outof said inner chamber.
 19. The test chamber as recited in claim 18,wherein said table has a follower that cooperates with a track attachedto said housing.
 20. The test chamber as recited in claim 19, whereinsaid follower is coupled to said wheel by a linkage arm, said track hasa step that rotates said follower and moves said wheel between retractedand extended positions.
 21. The test chamber as recited in claim 20,further comprising a brake coupled to a handle that allows movement ofsaid table when said handle is rotated.